This invention is generally related to methods and apparatus for the spectroscopic analysis of shock-compressed materials. More specifically, the present invention is related to the analysis of shock-compressed transparent liquids and solids by the use of coherent anti-Stokes Raman scattering (CARS) spectoscopy.
A continuing project of the Los Alamos National Laboratory is the study of the chemical and physical characteristics of materials at temperatures and pressures approaching those which exist in detonations of high explosives. Such characteristics are useful for determining equations of state for such materials and for predicting the behavior of explosives in various environments and configurations.
For many purposes, it is sufficient and useful to determine the physical and chemical characteristics of materials which are shock-compressed to high temperatures and pressures by mechanical means rather than by the use of explosives, thus enabling simpler and safer experiments to be conducted under controlled conditions which to some extent simulate the conditions in an explosive detonation. The present invention is directed to a novel spectroscopic technique which is particularly useful for observing and determining vibrational frequencies of shock-compressed organic liquids.
Light impinging on a molecule is ordinarily partially scattered by an elastic scattering process known as Rayleigh scattering. However, a small fraction of the light may undergo inelastic, or Raman, scattering. In Raman scattering a portion of the energy of the impinging photon is absorbed by the molecule, resulting in the scattered photon having a lower energy (and longer wavelength) than that of the impinging photon. In both of these processes, the molecule is excited by the impinging photon to a virtual energy level. In Rayleigh scattering the molecule decays back to the initial energy level, whereas in Raman scattering the molecule decays to an excited vibrational level which is typically the v=1 vibrational state. The difference in energy between the impinging photon and the emitted Raman photon is equal to the energy difference between the ground and v=1 vibrational states.
In the technique known as coherent anti-Stokes Raman spectroscopy, two laser beams are utilized. One laser beam is used to excite the molecule to the virtual energy level and the other beam is used to stimulate decay of the molecule from the virtual energy level to the excited vibrational state, resulting in Raman emission from the excited molecule. The first laser beam is referred to as the pump beam and the second beam is referred to as the Stokes beam. The function of the Stokes beam is to stimulate Raman emission from the population of molecules in the excited virtual energy state and thereby create a coherent population of molecules in the excited vibrational state. This population of molecules in the excited vibrational level is then susceptible to further coherent excitation by the pump beam, which excites them to a second virtual energy level that is higher than the virtual energy level which was attained by pumping of the molecule from the ground state. The molecules that are excited to the second, higher virtual energy level can then decay to the ground energy state. This latter decay occurs by coherent emission, resulting in a laser beam (known as the anti-Stokes beam) which has an energy (and frequency) that is higher than that of either the pump beam of the Stokes beam. This sequence of events actually occurs simultaneously through a four-wave mixing process. From the observed frequency of the anti-Stokes beam, the energy level of the v=1 vibrational state can be determined from the relationship .omega..sub.r =.omega..sub.as -.omega..sub.p, where .omega..sub.r is the frequency for the transition from the ground vibrational state to the v=1 vibrational level, .omega..sub.p is the frequency of the pump beam, and .omega..sub.as is the frequency of the anti-Stokes beam. This technique is generally known as coherent anti-Stokes Raman spectroscopy (CARS).
As discussed further below, the present invention is essentially an application of a variation of the above-described technique, which is known as broadband coherent anti-Stokes Raman scattering spectroscopy (broadband CARS), to shock-compressed condensed-phase materials. The applicants and others have recently applied a related technique known as backward-stimulated Raman scattering (BSRS) to vibrational frequency shifts in shock-compressed liquids. However, the latter technique suffers from a relative disadvantage in that only the Raman-active vibrational mode with the largest transition cross section undergoes stimulated scattering. This precludes detection of more than one chemical species or more than one vibrational mode in a single species. In addition, for some molecules the incident power density required to induce scattering is large enough to damage optical components located near focal points.